1. Field of the Invention
The present invention concerns a method of operating an active matrix liquid crystal display device having a liquid crystal layer between a pair of substrates and at least one active device to each of picture element electrodes on at least one of the substrates, so that a driving signal applied to one picture element electrode in one frame cycle is always on a side of a voltage having an identical polarity.
2. Discussion of the Background
So far, in an active matrix liquid crystal display device, cross talk occurs by a driving signal applied during a non-selection period and scattering in a display appears. Particularly, in a frame inversion type, since the selection time on the first scanning line is different from that on the n.sub.th scanning line, r.m.s. voltage values to the liquid crystal layer are different even if an identical data signal is inputted and scattering in a display appears frequently.
FIG. 7 shows various waveforms in the conventional frame inversion operation method in the active matrix device. In the drawing, the solid line indicates a driving signal waveform, while the broken line and the hatched portion indicate the waveform of the r.m.s. voltage applied to the liquid crystal layer. As apparent from the drawing, the r.m.s. voltage is different on end of the scanning lines even if the data voltage of the same condition is inputted, and scattering in a display occurs frequently.
In the line inversion operation method, the data signal swings between "+" and "-" regions in the period other than the selection period, which not only increases the driving voltage but also makes the condition for the signal input pattern being different greatly depending on the picture display pattern, and also causes scattering in the display and worsen the contrast as compared with that in the frame inversion operation method. As can be seen from the drawing, although the conditions are identical for all of the scanning lines during the selection period and there is no scattering in the display, the driving signal swings between "+" and "-" regions during the non-selection period, which hinders to hold the voltage and increase the driving voltage. Further, since there is a large leak current, high contrast can not be obtained.
Further, although it is highly desirable to use polymer dispersed liquid crystals as the liquid crystals in view of its performance, such liquid crystals require a high driving voltage and, accordingly, it is difficult for active matrix driving. Although there has been an example using a varister capable of withstanding a high voltage as the switching device, its driving is difficult since there is no high voltage resistant driving IC and in view of the stability of the switching device.
The present inventors have made an earnest study for developing an active matrix liquid crystal display device with no scattering in the display, of a high contrast and operable at a low driving voltage and, as a result, have accomplished the present invention based on the finding that the object can be attained by keeping a driving signal applied to a picture element electrode, each having an active device, during one frame cycle always on a side of a voltage having an identical polarity.